This invention relates to an apparatus for freezing the set point pressure on a hydraulic choke when air pressure in the choke drops below a normal operating range and for returning the choke to normal operation when air pressure returns to the normal range.
There are many applications in which there is a need to control the back pressure of a fluid flowing in a system. For example, in the drilling of oil wells, it is customary to suspend a drill pipe in the well bore with a bit on the lower end thereof and, as the bit is rotated, to circulate a drilling fluid, such as a drilling mud, down through the interior of the drill string, out through the bit, and up the annulus of the well bore to the surface. This fluid circulation is maintained for the purpose of removing cuttings from the well bore, for cooling the bit, and for maintaining hydrostatic pressure in the well bore to control formation gases and prevent blowouts and the like. In those cases where the weight of the drilling mud is not sufficient to contain the bottom hole pressure in the well, it becomes necessary to apply additional back pressure on the drilling mud at the surface to compensate for the lack of hydrostatic head and thereby keep the well under control. Thus, in some instances, a back pressure control device is mounted in the return flow line for the drilling fluid.
Back pressure control devices are also necessary for controlling “kicks” in the system caused by the intrusion of salt water or formation gases into the drilling fluid which may lead to a blowout condition. In these situations, sufficient additional back pressure must be imposed on the drilling fluid such that the formation fluid is contained and the well controlled until heavier fluid or mud can be circulated down the drill string and up the annulus to kill the well. It is also desirable to avoid the creation of excessive back pressures which could cause drill string to stick, or cause damage to the formation, the well casing, or the well head equipment.
However, maintenance of an optimum back pressure on the drilling fluid is complicated by variations in certain characteristics of the drilling fluid as it passes through the back pressure control device. For example, the density of the fluid can be altered by the introduction of debris or formation gases, and/or the temperature and volume of the fluid entering the control device can change. Therefore, the desired back pressure will not be achieved until appropriate changes have been made in the throttling of the drilling fluid in response to these changed conditions. Conventional devices generally require manual control of and adjustments to the choking device orifice to maintain the desired back pressure. However, manual control of the throttling device involves a lag time and generally is inexact.
U.S. Pat. Nos. 4,355,784 and 6,253,787, assigned to the assignee of the present application, disclose apparatuses and methods for controlling back pressure of drilling fluid in the above environment which address the problems set forth above. According to these arrangements, a balanced choke device moves in a housing to control the flow and back pressure of the drilling fluid. One end of the choke device is exposed to the pressure of the drilling fluid and its other end is exposed to the pressure of a control fluid. In operation, a “throttling” area defined between an end of a choke member and a choke seat, is greater than the area of a discharge passage. Thus, there is no back pressure imparted to the drilling fluid as it passes through an inlet passage and the discharge passage and discharges from the housing for recirculation. When it is desired to exert a back pressure on the drilling fluid in the passage, control fluid from an external source is introduced at a predetermined set point pressure into the passage and thus passes into chambers where it applies pressure to the back side of the choke member. As drilling fluid passes from the well being drilled into and through the inlet passage and into the housing, the effective opening or passage between the inlet passage and the discharge passage is controlled by the axial movement of the choke member relative to the choke seat. However, when a well has been drilled and is ready for production, alternative equipment is used than that used for drilling the well.
Typically, the pressure control apparatus is removed when the well is ready for production and replaced with BOPs. As the pressure control apparatus such as that described above is removed, the source of external fluid is disconnected. The hydrostatic pressure of the mud circulation system should be in equilibrium before the well is killed. For a variety of reasons, including the failure of monitoring devices and operator error, pressure in the throttling area may be present. When the external fluid source is disconnected, the choke member is no longer counterbalanced by the control fluid and the increased pressure is dissipated through the choke and any downstream equipment, causing potential damage to the equipment and risk of injury or death to people nearby.
Therefore, what is needed is a method and system for maintaining back pressure on the drilling fluid when the external fluid source is disengaged along with a warning system to alert an operator if the drilling fluid in the throttling area is under pressure.
Thus, a safety mechanism has been invented to prevent the immediate loss of pressure through the pressure control apparatus when the external fluid source is disconnected.